linux-kernel - Re: [PATCHSET v6 0/4] Split iowait into two states

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Message-ID: <CAJZ5v0ga0FJtgQhEWzq6qYDjDz=TjiAzr4ApPAT7U_GosG3m9w@mail.gmail.com>
Date: Thu, 5 Sep 2024 12:40:17 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Christian Loehle <christian.loehle@....com>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>, Peter Zijlstra <peterz@...radead.org>, 
	Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org, tglx@...utronix.de, 
	daniel.lezcano@...aro.org, linux-pm@...r.kernel.org
Subject: Re: [PATCHSET v6 0/4] Split iowait into two states

On Thu, Sep 5, 2024 at 11:29 AM Christian Loehle
<christian.loehle@....com> wrote:
>
> On 9/4/24 16:18, Rafael J. Wysocki wrote:
> > On Wed, Sep 4, 2024 at 4:42 PM Rafael J. Wysocki <rafael@...nel.org> wrote:
> >>
> >> On Wed, Sep 4, 2024 at 4:28 PM Peter Zijlstra <peterz@...radead.org> wrote:
> >>>
> >>> On Mon, Aug 19, 2024 at 09:39:45AM -0600, Jens Axboe wrote:
> >>>> Hi,
> >>>>
> >>>> This is v6 of the patchset where the current in_iowait state is split
> >>>> into two parts:
> >>>>
> >>>> 1) The "task is sleeping waiting on IO", and would like cpufreq goodness
> >>>>    in terms of sleep and wakeup latencies.
> >>>> 2) The above, and also accounted as such in the iowait stats.
> >>>>
> >>>> The current ->in_iowait covers both, this series splits it into two types
> >>>> of state so that each can be controlled seperately.
> >>>
> >>> Yeah, but *WHY* !?!? I have some vague memories from last time around,
> >>> but patches should really keep this information.
> >>>
> >>>> Patches 1..3 are prep patches, changing the type of
> >>>> task_struct->nr_iowait and adding helpers to manipulate the iowait counts.
> >>>>
> >>>> Patch 4 does the actual splitting.
> >>>>
> >>>> This has been sitting for a while, would be nice to get this queued up
> >>>> for 6.12. Comments welcome!
> >>>
> >>> Ufff, and all this because menu-governor does something insane :-(
> >>>
> >>> Rafael, why can't we simply remove this from menu?
> >>
> >> Same reason as before: people use it and refuse to stop.
> >>
> >> But this is mostly about the schedutil cpufreq governor that uses
> >> iowait boosting.
> >
> > To be more precise, there are two different uses of "iowait" in PM.
> >
> > One is the nr_iowait_cpu() call in menu_select() and the result of it
> > is used for two purposes: (1) select different sets of statistics
> > depending on whether or not this number is zero and (2) set a limit
> > for the idle state's exit latency that depends on this number (but
> > note that it only takes effect when the "iowait" statistics are used
> > in the first place).  Both of these are arguably questionable and it
> > is unclear to me whether or not they actually help and how much.
>
> So from my perspective it doesn't, not significantly to justify it's
> existence anyway. Either it doesn't actually matter for menu, or teo
> is able to compete / outperform without relying on it.

Thanks for this feedback!

I'm actually going to try to remove that stuff from menu and see if
anyone cries bloody murder.

> Some caution is advised though this really depends on:
> - Which idle states are available for the kernel to select.
> - How accurate the kernel's view of the idle states is.
>
> Both varies wildly.

True, but let's see what the feedback is.

> > The other use is boosting CPU frequency in schedutil and intel_pstate
> > if SCHED_CPUFREQ_IOWAIT is passed to them which in turn depends on the
> > p->in_iowait value in enqueue_task_fair().
> >
> > AFAICS, the latter makes a major difference.
>
>
> Indeed, fortunately the impact is quite limited here.
> But please, Rafael, Jens and Peter, feel free to share your comments
> over here too:
>
> https://lore.kernel.org/lkml/20240905092645.2885200-1-christian.loehle@arm.com/

I will.

Thanks!